Analysis of diesel combustion in four-stroke marine engines : an integrated CFD and reduced chemical kinetics approach

The present thesis aims at characterising and understanding flow and combustion processes in four-stroke marine engines by means of detailed Computational Fluid Dynamics (CFD) modelling. In particular, two dual-fuel engines operating in the diesel mode are considered: Lister LV1 and Wärtilä 50DF. A systematic approach is followed, consisting in: (a) Characterising the spray dynamics in a constant volume chamber, and adapting the Cascade Atomisation Breakup (CAB) spray model, for conditions relevant to operation of the engines considered in the present study. (b) Adapting a tool developed by means of coupling the chemical kinetics CHEMKIN-II code with the KIVA-3vr2 CFD code, thus enabling CFD simulations with reliable chemistry. (c) Performing CFD simulations of flow and combustion in the two engines for operation in the 80% load for Lister LV1, and in the full load range for Wärtilä 50DF, and comparing results against one-step chemistry simulations, as well as against respective experimental data. Thus, to author’s knowledge, the present thesis reports the first CFD studies in marine engines operating in the diesel mode, including realistic combustion chemistry. The computational results demonstrate that the spray breakup corresponds to the catastrophic regime. Adaptation of the CAB model has yielded values of model constants in a range that does not considerably deviate from relevant literature studies. Two validated reduced-order chemical kinetic mechanisms of n-heptane combustion have been implemented in the course of the present CFD studies: (i) Patel et al. (2004), with 29 species and 52 elementary reactions, and (ii) Ra and Reitz (2008), with 45 species and 142 elementary reactions. The two mechanisms have been supplemented with a NOx sub mechanism. The main findings of the present study can be summarised as follows: - Ignition delay times are in good agreement with chemical kinetics simulations using realistic chemistry. - For simulations with reduced order chemistry, fuel disintegration nearly terminates at the end of injection, in contrast to results of the one-step approach. On the other hand, similarities in Rate of Heat Release Rates are attained for the two approaches. - The evolution of important species can differ considerably between the two reduced mechanisms. - The distribution of main pollutants bears similarities between the two reduced mechanisms, and may differ significantly from the one-step approach. While the present study demonstrates that using reduced order chemistry is essential for characterising engine aerothermochemistry albeit at a significant increase in computational cost, the one-step approach is shown to still be valid as an engineering tool, providing a basic characterisation of flow and combustion, which can be useful in the frame of marine engine development.The present thesis aims at characterising and understanding flow and combustion processes in four-stroke marine engines by means of detailed Computational Fluid Dynamics (CFD) modelling. In particular, two dual-fuel engines operating in the diesel mode are considered: Lister LV1 and Wärtilä 50DF. A systematic approach is followed, consisting in: (a) Characterising the spray dynamics in a constant volume chamber, and adapting the Cascade Atomisation Breakup (CAB) spray model, for conditions relevant to operation of the engines considered in the present study. (b) Adapting a tool developed by means of coupling the chemical kinetics CHEMKIN-II code with the KIVA-3vr2 CFD code, thus enabling CFD simulations with reliable chemistry. (c) Performing CFD simulations of flow and combustion in the two engines for operation in the 80% load for Lister LV1, and in the full load range for Wärtilä 50DF, and comparing results against one-step chemistry simulations, as well as against respective experimental data. Thus, to author’s knowledge, the present thesis reports the first CFD studies in marine engines operating in the diesel mode, including realistic combustion chemistry. The computational results demonstrate that the spray breakup corresponds to the catastrophic regime. Adaptation of the CAB model has yielded values of model constants in a range that does not considerably deviate from relevant literature studies. Two validated reduced-order chemical kinetic mechanisms of n-heptane combustion have been implemented in the course of the present CFD studies: (i) Patel et al. (2004), with 29 species and 52 elementary reactions, and (ii) Ra and Reitz (2008), with 45 species and 142 elementary reactions. The two mechanisms have been supplemented with a NOx sub mechanism. The main findings of the present study can be summarised as follows: - Ignition delay times are in good agreement with chemical kinetics simulations using realistic chemistry. - For simulations with reduced order chemistry, fuel disintegration nearly terminates at the end of injection, in contrast to results of the one-step approach. On the other hand, similarities in Rate of Heat Release Rates are attained for the two approaches. - The evolution of important species can differ considerably between the two reduced mechanisms. - The distribution of main pollutants bears similarities between the two reduced mechanisms, and may differ significantly from the one-step approach. While the present study demonstrates that using reduced order chemistry is essential for characterising engine aerothermochemistry albeit at a significant increase in computational cost, the one-step approach is shown to still be valid as an engineering tool, providing a basic characterisation of flow and combustion, which can be useful in the frame of marine engine development